title: Two-particle density matrix cumulant of coupled cluster theory.
abstract: A two-particle density matrix obtained from the expectation value expression for the coupled cluster wave function is separated into the antisymmetrized product of the one-particle density matrices and the remaining cumulant part. It is demonstrated that the proposed formula for the coupled cluster cumulant is a valid cumulant expression, since it is a connected, and therefore size-extensive quantity. It is also shown that the density matrices from coupled cluster gradient theory, when used to define a cumulant, result in the expression containing disconnected terms. The proposed formulation of the coupled cluster cumulant makes it easy to develop size-extensive truncation schemes. As an example, explicit equations for the cumulant at the coupled cluster single, double and triple excitation level are presented.
authors.affiliation: Faculty of Chemistry, University of Warsaw, Warsaw, Poland. tatiana.korona@chem.uw.edu.pl
authors.forename: Tatiana
authors.initials: T
authors.lastname: Korona
mesh_terms: D016000:Cluster Analysis; D011789:Quantum Theory